There already exist methods for detecting the stall state of the rotor of a stepper motor. They use the fact that a change in the movement of the motor will cause a variation of the BEMF (Back electro-magnetic force) and thus also the induced voltage. The current in the active coil is also subject to change. The known methods for stall detection differ in their measurement methods and the way the measured signals are analyzed.
U.S. Pat. No. 5,032,781 and U.S. Pat. No. 5,287,050 describe a method and circuit for operating a stepping motor having several windings, wherein a movement of the stepping motor induces a voltage in a winding which is not required at the time for generating a propulsive force. This induced voltage is evaluated using a threshold detection element. After the stepping motor is connected, the stepping motor is operated in a predetermined direction, whereupon a stopping of the stepping motor is detected by the absence of the induced voltage. A memory containing the instantaneous position of the stepping motor is then set to a predetermined value.
In the stall state detection apparatus disclosed in U.S. Pat. No. 6,861,817 B2 a stepped rotary motion is imparted to the rotor of a stepping motor by alternately driving at least first and second coils which interact with a plurality of magnetic poles on the rotor. When each of the coils transitions from a driven to a non-driven state, the continued motion of the rotor causes a back electro-motive force to be generated in the coil. The electro-motive forces produced by the coils are rectified, integrated, and then compared with a threshold to determine if a motor stall condition exists.
The detection device disclosed in U.S. Pat. No. 7,224,140 B2 detects a stall in a stepper motor by determining a motor winding current for each stepper pulse and determining if the winding current of a particular stepper pulse meets predetermined criteria. The motor winding current may be determined by measuring a voltage across an ON field effect transistor during a stepper pulse and calculating a winding current using an assumed ON field effect transistor resistance. The predetermined criteria may by a calculated motor winding current greater than a predetermined threshold, greater than prior pulse by more than a predetermined threshold or greater than a prior pulse by more than a predetermined factor.